Haber-Bosch method for ammonia

Making ammonia from nitrogen and hydrogen

1. Magnetic properties and catalytic activity

2. Ammonia from air

Air is mostly nitrogen. Factories turn it into ammonia,

NH3 NH X_{3}

, by the Haber-Bosch method.

Pick the right answer

Predict: which gas must react with nitrogen to give

NH3 NH X_{3}

A. Hydrogen
B. Oxygen
C. No other gas: nitrogen alone becomes ammonia

الشرح

NH3 NH X_{3}

is made of nitrogen and hydrogen, so nitrogen must react with hydrogen:

N2 + 3H2 \to 2NH3 N X_{2} + 3 H X_{2} 2 NH X_{3}

3. The book's equation

gatheredN2(g) + 3H2(g) →[500\,^C / 200\,atm,\ Fe]\\ 2NH3(g)gathered

Preparation of ammonia by the Haber-Bosch method (book p.15)

Read it: nitrogen gas and hydrogen gas give ammonia gas. One

N2 N X_{2}

reacts with three

H2 H X_{2}

to give two

NH3 NH X_{3}

Over the arrow: 500 °C, a pressure of 200 atm, and iron, Fe, the catalyst. 200 atm (atmospheres) is 200 times normal air pressure.

4. Why iron is needed

The bond in

N2 N X_{2}

is very strong. Breaking it needs a high activation energy, so the reaction is very slow.

Iron, a transition element, lowers the activation energy. So ammonia forms fast enough for a factory.

	Without iron	With iron (catalyst)
Activation energy	high	lowered
Rate of forming ammonia	very slow	fast enough for a factory

The same reaction without and with iron

5. Use the equation

Pick the right answer

In the Haber-Bosch method, 6 molecules of

H2 H X_{2}

react completely with enough

N2 N X_{2}

. How many molecules of

NH3 NH X_{3}

form?

A. 4
B. 6
C. 2
D. 12

الشرح

N2 + 3H2 \to 2NH3 N X_{2} + 3 H X_{2} 2 NH X_{3}

, three

H2 H X_{2}

give two

NH3 NH X_{3}

. Six

H2 H X_{2}

are two sets of three, so they give 4

NH3 NH X_{3}

Pick the right answer

Which statement about the iron in the Haber-Bosch method is correct?

A. It lowers the activation energy and is not used up
B. It reacts with nitrogen to form ammonia
C. It increases the amount of ammonia formed

الشرح

Iron is the catalyst, written over the arrow: it lowers the activation energy, so the reaction goes faster, and it is not used up. It is not part of

NH3 NH X_{3}

and gives no extra product.

6. Summary

Ammonia is made industrially from nitrogen and hydrogen by the Haber-Bosch method.
$gatheredN2(g) + 3H2(g) →[500\,^C / 200\,atm,\ Fe]\\ 2NH3(g)gathered$
Iron is the catalyst: it lowers the activation energy of this slow reaction.

Can you explain why iron is written over the arrow and not as a reactant?

Reveal

Iron is the catalyst, not a reactant: it lowers the activation energy and speeds up the reaction, but it is not used up and gives no extra ammonia. That is why it sits over the arrow.

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